1. Field of the Invention
The invention relates to a capacitor microphone having a housing, having a first membrane likewise a backplate electrode associated with this membrane, and an opening through which sound can reach the membrane.
2. Description of the Related Art
A known capacitor microphone is of the MKE 2-microphone type by Sennheiser electronic GmbH & Co. This MKE 2-microphone is a permanent polarized capacitor microphone, which as a high grade, small attachable microphone having a diameter of about 4-6 mm, is inserted everywhere where other attachable microphones are too conspicuous due to their greater dimensions. Such very small microphones of the highest quality are used particularly in concert performances, musicals or in other showbusiness where the artist sings or speaks in addition to the theatrical or dance performance, and the microphone is attached well hidden, to the artist's body, for example in their hair or inside the costume, being correspondingly aligned to the artist's mouth.
The MKE 2 fulfils the highest claims as regards quality of tone and sturdiness, and is suitable for the transmission of speech and instrument pick-up in all areas of live sound transmission technology. The apparatus can be connected directly to apparatus having 12-14 volts phantom power supply, and is relatively non-sensitive to impact sound, and has at its disposal a very linear frequency response, this being very important for a faithful recording.
It can happen in unfavourable circumstances that sweat penetrates the microphone capsule MKE 2 and distorts it, especially if the artist perspires heavily. In this connection one must know that a capacitor microphone is a pressure microphone which is usually insensitive to high atmospheric moisture, as the air exchange with a sensitive electret is itself interrupted through the membrane in front of the back plate electrode. The atmospheric moisture inside the microphone or microphone housing only equates very slowly to the outside atmospheric moisture, as the membrane is vapour-permeable in general. If the microphone capsule is manufactured clean, this is not a problem. Only the penetration of salts, e.g. electrolytic fluids such as are contained in human sweat is critical. They would immediately discharge the electret sheet to the backplate electrode. Both known microphones are of the MKE 2 type, and as with all other capacitor microphones also, an acoustic orifice is provided as a sound passage opening through which the incoming sound arrives in an outer area inside the microphone and finally strikes the membrane. The membrane is arranged on a membrane ring, and the sweat cannot force past the membrane ring itself as it is located in a silicone seal.
The sweat is subsequently sucked through a very small hole (opening) having a diameter of only 10-30 .mu.m and which is arranged in the membrane in the critical air gap between the membrane and the backplate electrode. This leads to the discharge of the electret sheet. The above-named small membrane opening is provided with capacitor microphones for equalising pressure, so that the membrane does not "cling" to the backplate electrode on air pressure oscillations, which can lead to damage on the one hand, and on the other, to undesired receiving noises. Independent of the small membrane opening location, it can hardly be avoided that at some point, sweat will arrive in the air gap between the membrane and the backplate electrode, and lead to the discharge of the electret sheet.
The sweat problem has been known for a long time and has until now been fought against, for example in that a preferably water-repellent, vapour and sound permeable polyester fleece is arranged in front of the microphone housing acoustic opening. In addition the whole microphone capsule together with the soldering joints are hermetically sprayed so as to avoid sweat penetration to other parts of the microphone as well.
However, it has been shown that in spite of the above measures, no completely reliable sweat deflection is possible inside the microphone, as sweat arrives in the capacitor microphone time and again in unfavourable circumstances, and can lead to a microphone cut-out. Above all, the known materials introduced into the microphone capsule or the membrane are repellent as regards distilled water. However, after a certain time, they permit sweat to penetrate due to its small surface tension, and thus do not fulfil the desired requirements, which in the worst case could lead to a complete microphone cut-out.